1. Field of the Invention
This invention relates generally to the downhole investigation of subterranean formations. More particularly, this invention relates to sampling through perforations in a wellbore penetrating the subterranean formation.
2. Background Art
Historically, wells have been drilled to seek out downhole reservoirs containing highly desirable fluids, such as oil, gas or water. The wells may be located on land or over waterbeds and extend downhole into subterranean formations. In the search for oil and gas reserves, new wells are often drilled and tested. The wellbore may remain “open” after drilling, or be provided with a casing (otherwise known as a liner) to form a “cased” wellbore. A cased wellbore is created by inserting a tubular steel casing into an open wellbore and pumping cement downhole to secure the casing in place in the wellbore. The cement is employed on the outside of the casing to hold the casing in place and to provide a degree of structural integrity and a seal between the formation and the casing.
Various tests are typically performed on open wellbores to analyze surrounding formations for the presence of oil and gas. Once the casing is installed, the ability to perform tests is limited by the steel casing. It is estimated that there are approximately 200 cased wells which are considered for abandonment each year in North America, which adds to the thousands of wells that are already idle. These abandoned wells have been determined to no longer produce oil and gas in necessary quantities to be economically profitable. However, the majority of these wells were drilled in the late 1960's and 1970's and logged using techniques that are primitive by today's standards. Thus, recent research has uncovered evidence that many of these abandoned wells contain large amounts of recoverable natural gas and oil (perhaps as much as 100 to 200 trillion cubic feet) that have been missed by conventional production techniques. Because the majority of the field development costs such as drilling, casing and cementing have already been incurred for these wells, the exploitation of these wells to produce oil and natural gas resources could prove to be an inexpensive venture that would increase production of hydrocarbons and gas. It is, therefore, desirable to perform additional tests on such cased wellbores.
In order to perform various tests on a cased wellbore to determine whether the well is a good candidate for production, it is often necessary to perforate the casing to investigate the formation surrounding the wellbore. One such commercially used perforation technique employs a tool which can be lowered on a wireline to a cased section of a borehole, the tool including a shaped explosive charge for perforating the casing, and testing and sampling devices for measuring hydraulic parameters of the environment behind the casing and/or for taking samples of fluids from said environment. Perforations may also be used in open wellbores, for example, to facilitate the exploration of the surrounding formation and/or the flow of fluid from the formation into the wellbore.
Various techniques have been developed to create perforations in wellbores. For example, U.S. Pat. No. 5,195,588 issued to Dave and U.S. Pat. No. 5,692,565 issued to MacDougall et al., both assigned to the assignee of the present invention, disclose techniques for perforating a wellbore. These patents also provide techniques for plugging a wellbore after the perforation is created to stop the flow of fluid through the casing and into the wellbore.
While the advances in perforation techniques have assisted in the analysis of open and cased wellbores, it has been discovered that some perforations may become obstructed by debris. This debris may prevent the passage of fluids and/or tools through the perforation. Additionally, debris, such as drilling fluids, mud, dirt and other contaminants, may pollute the sampling or testing process and corrupt the test results.
Techniques have also been developed to prevent contamination of samples collected during the sampling process. For example, U.S. Pat. No. 4,495,073 to Beimgraben, U.S. Pat. No. 5,379,852 to Strange, Jr. and U.S. Pat. No. 5,377,750 to Arterbury each disclose filtering techniques for preventing downhole drilling fluids from contaminating samples. However, these techniques fail to address the problem of contamination and debris in the perforation.
To address problems, such as obstructions and contamination encountered with perforations, there remains a need to develop techniques to remove debris. It is desirable that such techniques reduce the contamination of fluids sampled from a perforation and/or prevent clogging of the perforation. It is also desirable that such techniques be usable in conjunction with perforating, testing, sampling and/or plugging operations. Such a technique should, among others, improve the quality of the sample, reduce the potential for debris to flow into the perforation, reduce the likelihood of clogging the perforation, reduce contamination in the sample, reduce contamination in the downhole tool and/or provide other advantages.